We have developed a technique to produce, manipulate and trap Rb2 molecules with a single optical beam. This beam is generated by a high-power fiber amplifier (50 W of total output power) in the 1060 to 1070 nm range, which is seeded by two light sources: a sharp-band laser and a broadband superluminescent diode. The laser source is tuned to produce Rb2 molecules from an ultracold 85Rb sample via photoassociation. The broadband spectrum vibrationally cool the molecules by optical pumping. This source is spectrally shaped in order to populate the molecules at the fundamental vibrational state ν = 0. The molecular sample is probed by two-photon ionization, promoted by a pulsed dye laser in the 475 to 480 nm range. By scanning the photoassociation laser frequency, we have obtained a photoassociation spectrum of the ν\' = 138 state of the 0+u potential, confirming previous observations. We have also obtained two vibrational spectra of the molecules by varying the ionization laser frequency, in the presence and absence of the broadband source. The comparison between the two spectra, along with ab-initio data, provides evidences that the molecules are optically pumped to the fundamental vibrational state. / Desenvolvemos uma técnica para produzir, manipular e aprisional moléculas de Rb2 com um único feixe óptico. Esse feixe é gerado por um amplificador de alta potência (50 W de potência total de saída) na faixa de 1060 a 1070 nm, que é alimentado por duas fontes de luz: um laser banda estreita e um diodo superluminescente banda larga. O laser é sintonizado para produzir moléculas de Rb2 a partir de uma amostra ultrafria de 85Rb via fotoassociação. O espectro banda larga resfria vibracionalmente as moléculas, por bombeamento óptico. Essa fonte de luz tem seu espectro formatado de modo a deixar as moléculas em seu estado vibracional fundamental ν = 0. A amostra molecular é testada por ionização de dois fótons, promovida por um laser de diodo pulsado na faixa de 475 a 480 nm. Variando a frequência do laser de fotoassociação, obtivemos um espectro de fotoassociação do estado ν\' = 138 do potencial 0+u, confirmando observações anteriores. Obtivemos também dois espectros vibracionais das moléculas variando a frequência do laser de ionização, com e sem a fonte banda larga. Uma comparação entre os dois espectros, junto de dados ab-initio, fornece evidências de que as moléculas são opticamente bombeadas para o estado vibracional fundamental.

We have developed a technique to produce, manipulate and trap Rb2 molecules with a single optical beam. This beam is generated by a high-power fiber amplifier (50 W of total output power) in the 1060 to 1070 nm range, which is seeded by two light sources: a sharp-band laser and a broadband superluminescent diode. The laser source is tuned to produce Rb2 molecules from an ultracold 85Rb sample via photoassociation. The broadband spectrum vibrationally cool the molecules by optical pumping. This source is spectrally shaped in order to populate the molecules at the fundamental vibrational state ν = 0. The molecular sample is probed by two-photon ionization, promoted by a pulsed dye laser in the 475 to 480 nm range. By scanning the photoassociation laser frequency, we have obtained a photoassociation spectrum of the ν\' = 138 state of the 0+u potential, confirming previous observations. We have also obtained two vibrational spectra of the molecules by varying the ionization laser frequency, in the presence and absence of the broadband source. The comparison between the two spectra, along with ab-initio data, provides evidences that the molecules are optically pumped to the fundamental vibrational state. / Desenvolvemos uma técnica para produzir, manipular e aprisional moléculas de Rb2 com um único feixe óptico. Esse feixe é gerado por um amplificador de alta potência (50 W de potência total de saída) na faixa de 1060 a 1070 nm, que é alimentado por duas fontes de luz: um laser banda estreita e um diodo superluminescente banda larga. O laser é sintonizado para produzir moléculas de Rb2 a partir de uma amostra ultrafria de 85Rb via fotoassociação. O espectro banda larga resfria vibracionalmente as moléculas, por bombeamento óptico. Essa fonte de luz tem seu espectro formatado de modo a deixar as moléculas em seu estado vibracional fundamental ν = 0. A amostra molecular é testada por ionização de dois fótons, promovida por um laser de diodo pulsado na faixa de 475 a 480 nm. Variando a frequência do laser de fotoassociação, obtivemos um espectro de fotoassociação do estado ν\' = 138 do potencial 0+u, confirmando observações anteriores. Obtivemos também dois espectros vibracionais das moléculas variando a frequência do laser de ionização, com e sem a fonte banda larga. Uma comparação entre os dois espectros, junto de dados ab-initio, fornece evidências de que as moléculas são opticamente bombeadas para o estado vibracional fundamental.

Beaucoup d’expériences et d’applications utilisant des molécules froides nécessitent d’avoir un échantillon de molécules froides à la fois en translation, en vibration et en rotation. Cette thèse se situe dans la même thématique, elle a pour objectif la généralisation de la méthode du pompage optique qui a permis de refroidir la vibration des molécules de Césium par un laser large bande dont les fréquences correspondantes aux transitions partant du niveau vibrationnel fondamental ont été supprimées. Nous avons, par exemple, réalisé un transfert de la population moléculaire dans un niveau vibrationnel pré-sélectionné. Nous avons ensuite démontré que le refroidissement vibrationnel est aussi efficace avec une source de lumière non cohérente. Enfin, une technique de conversion électronique a été démontrée et qui a permis de transférer les molécules formées dans l’état électronique métastable a3Σu+ vers l’état électronique fondamental X1Σg+. Finalement, l’application de cette méthode sur la rotation a été étudié, les résultats préliminaires montrent que le processus marche efficacement ce qui ouvre des perspectives sur le refroidissement lasers des molécules. / Many experiments and applications using cold molecules need to have a sample of molecules cold in all degrees of freedom. My activity in the cold molecule experiment considered several extensions and generalizations of the vibrational cooling technique using a shaped broadband laser. The first extension realized was the transfer of the molecular population to any pre-selected vibrational level. Another extension was the realization of vibrational cooling and molecular population transfer with the use of a broadband, non-coherent, diode light source, instead of a femtosecond laser. Finally, we demonstrate an efficient technique which allows us to convert molecules initially formed in the triplet state (a3Σu+) into the ground electronic state (X1Σg+). The generalization of the vibrational cooling technique to include rotation has also been studied. Preliminary experiments considered for rotational cooling as well as more detail theoretical treatment has been performed. This open the way for more general laser technique apply to molecules.

Ces dix dernières années ont vu émerger des avancées technologiques majeures, nous permettant capturer une image instantanée des processus physique. L'amélioration systématique de la résolution temporelle de ces instants, grâce aux lasers (de différente sorte) aux impulsions ultracourtes, a joué un rôle important dans l'exploration des transitions de phases photo-induites dans différents matériaux, et leur potentiel applicatif. Néanmoins, ce progrès technologique incontestable a poussé à ses limites notre capacité de décrire les phénomènes hors-équilibre très complexes qui pilotent les transitions. Ils sont intrinsèquement multi-échelles dans le temps et dans l'espace, s'étalant de la femtoseconde aux plusieurs jours, et de la dimension atomique jusqu'à celle d'un cristal macroscopique. Les expériences résolues en temps permettent de séparer temporellement différents dégrées de liberté et les phénomènes pilotés par ceux-ci, au lieu d'observer seulement leur moyenne statistique. La première étape (processus photo-induit) de cette séquence temporelle est liée à l'absorption d'un photon, la deuxième (élastique) est pilotée par la dilation du volume macroscopique du matériau, et la troisième étape (thermique) est due aux effets de chaleur. Cette approche séquentielle offre de nouvelles possibilités pour mieux comprendre comment impacter les matériaux de façon contrôlée et efficace. Les lasers opérant dans le moyen infrarouge (mid IR) permettent de suivre le déroulement d'une transition de phase par le changement de vibrations des molécules/liaisons ciblées. Cette spécificité au site moléculaire combinée avec la résolution en temps ultracourte devrait ouvrir une nouvelle fenêtre d'observation des phénomènes qui échappaient aux études scientifique. Ce travail de thèse a commencé exactement dans cet esprit. L'effort majeur a été dédié à l'application de la spectroscopie mid IR ultrarapide aux matériaux présentant une conversion de l'état de spin, [Fe(3-MeO-SalEen)]2PF6 en particulier. La principale difficulté de ce travail consistait à décrypter le contenu spectral des molécules hors-équilibre. Nous avons découvert que l'approche utilisé dans les spectroscopies résolues en temps de plus haute énergie (UV/VIS) ne suffit pas pour étudier la problématique posé dans le cadre de cette thèse. Une nouvelle approche a été pensée pour modéliser les spectres résolus en temps, et celle-ci consistait à séparer la réponse spectrale en deux contributions : le transfert de poids spectral, et un décalage spectral. J'ai pu démontrer que ces deux contributions suivent sensiblement le changement d'état de spin, et la pression (dilatation du volume). L'analyse de données basée sur ce modèle, corrobore les résultats obtenus jusqu'alors avec d'autres techniques. Sur l'échelle de temps ultracourts, plus difficile à modéliser, nous avons pu néanmoins résoudre très clairement le refroidissement vibrationnelle (VC) de l'état électronique haut spin -chaud. A ma connaissance, ce phénomène dans un système solide présentant crossover de spin n'a jamais été observé directement. / The past few decades have seen great advancements in technology that allow us to capture the picture of a physical process, as the adage “seeing is believing” implies how people understand the world. The increasing temporal resolution of lasers played an important role in the study of materials, among which materials exhibiting photo-induced phase transition are of great importance thanks to their potential for future applications. However, as we proceed further and further in the investigation of the mechanism of phase transition, we found ourselves confronted with the very complex nature of phase transition dynamics. It is intrinsically multi-scale in time and space, from femtosecond to days and from atomic dimensions to macroscopic distances. Time resolved experiments disentangle different degrees of freedom and different phenomena in a step-like manner, rather than providing a statistical average. The first step is photo induced due to absorption of photons, the second step (elastic step) is pressure induced due to volume dilation, and the third step is temperature induced due to dissipation of heat. This step-like approach offers an opportunity to understand the mechanism, so that we could effectively impact the materials and possibly control phase transition. Mid IR lasers have a unique advantage of monitoring phase transition through vibrational modes on specific molecular sites. Implementing ultrafast mid IR spectroscopy in phase transition materials should be therefore very insightful in discovering new phenomena and revealing hidden mechanism. This PhD project, focusing on mid IR technique, started exactly in this context. The main effort is dedicated to the application of ultrafast mid IR spectroscopy to the spin crossover solids, [Fe(3-MeO-SalEen)]2PF6. The major challenge in this work was to comprehend the shape of transient mid IR spectra. We found out that this is conceptually different from the experiences accumulated in UV/VIS spectroscopy. A suitable model had to be developed, separating the transient IR spectra into two contributions: spectral weight transfer and spectral shift. I demonstrated that these two components are sensitive to the spin change and pressure effect (volume dilation), respectively. Data analysis based on the new model shows consistency with previously published results. On the ultrafast timescale, more difficult to fit our model to, vibrational cooling (VC) of electronically hot HS state has been very well resolved. To the best of my knowledge, direct observation of VC in solid state SCO compound by IR spectroscopy has not been reported earlier.

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